1. Field of the Invention
The present invention relates generally to interference pigments and foils. More particularly, the present invention relates to multilayer color shifting pigment flakes and foils which have titanium-containing absorber layers.
2. Background Technology
Various color shifting pigments, colorants, and foils have been developed for a wide variety of applications. For example, color shifting pigments have been used in applications such as cosmetics, inks, coating materials, ornaments, ceramics, automobile paints, anti-counterfeiting hot stamps and anti-counterfeiting inks for security documents and currency. Such pigments, colorants, and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted.
The color shifting properties of the pigments and foils can be controlled through proper design of the optical thin films used to form the flake or foil coating structure. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to the absorption characteristics of a material which are selectively enhanced at particular wavelengths from the interference phenomena.
Various approaches have been used to achieve such color shifting effects. For example, small multilayer flakes, typically composed of multiple layers of thin films, are dispersed throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Such flakes may optionally be overcoated to achieve desired colors and optical effects. Another approach is to encapsulate small metallic or silicatic substrates with varying layers and then disperse the encapsulated substrates throughout a medium such as paint or ink. Additionally, foils composed of multiple layers of thin films on a substrate material have been made.
One manner of producing a multilayer thin film structure is by forming it on a flexible web material with a release layer thereon. The various layers are deposited on the web by methods well known in the art of forming thin coating structures, such as PVD, sputtering, or the like. The multilayer thin film structure is then removed from the web material as thin film color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. In addition to the color shifting flakes, additives can be added to the inks or paints to obtain desired color shifting results.
Color shifting pigments or foils are formed from a multilayer thin film structure that includes the same basic layers. These include an absorber layer(s), a dielectric layer(s), and optionally a reflector layer, in varying layer orders. The coatings can be formed to have a symmetrical multilayer thin film structure, such as:                absorber/dielectric/reflector/dielectric/absorber; or absorber/dielectric/absorber. Coatings can also be formed to have an asymmetrical multilayer thin film structure, such as:        absorber/dielectric/reflector.        
For example, U.S. Pat. No. 5,135,812 to Phillips et al. discloses optically variable thin film flakes having several different configurations of layers such as transparent dielectric and semi-transparent metallic layered stacks. In U.S. Pat. No. 5,278,590 to Phillips et al., incorporated by reference herein, a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same composition and thickness, and a dielectric spacer layer located between the first and second absorber layers.
Color shifting platelets for use in paints are disclosed in U.S. Pat. No. 5,571,624 to Phillips et al., which is incorporated by reference herein. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. An opaque reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer of the same material and thickness as the first dielectric layer. A second semi-opaque layer of the same material and thickness as the first semi-opaque layer is formed on the second dielectric layer.
Interference pigments having titanium dioxide layers have been previously produced. For example, U.S. Pat. No. 5,116,664 to Kimura et al. discloses a pigment that is made by coating a first layer of TiO2 onto mica followed by coating the TiO2 layer with powder particles of at least one of the metals cobalt, nickel, copper, zinc, tin, gold, and silver. The metallic powder layer is deposited by an electroless wet chemical process. Electron micrographs showed that these particles are in the form of finely divided rods.
Interference pigments incorporating titanium oxide layers are disclosed in U.S. Pat. Nos. 5,364,467 to Schmid et al. and 5,573,584 Ostertag et al. Each of these patents teaches colorless, non-absorbing TiO2 layers or selectively absorbing metal oxide materials for overcoating platelet-like silicatic substrates (micas, talc or glass flakes) or platelet-like metallic substrates.
U.S. Pat. No. 5,607,504 to Schmid et al. discloses pigments with titanium(III) oxide, titanium oxynitride, and titanium nitride coatings, formed by the reduction of titanium dioxide. The pigment particles are composed of various metal substrates upon which is deposited a selectively absorbing coating of titanium oxynitrides and titanium nitride with titanium dioxide and titanium III oxide by using hydrolytic decomposition of titanium tetraisopropoxide or titanium tetrachloride and subsequent reduction with ammonia.
In U.S. Pat. No. 4,978,394 to Ostertag, metal oxide coated aluminum pigments are disclosed, which include a substrate of platelet-like aluminum coated with layers of titanium oxides of different thicknesses. The titanium oxide layers are formed by a chemical vapor deposition process whereby titanium tetrachloride is reacted with water vapor. Optionally, the titanium dioxide layer can then be reduced to form TiO, TiN, or titanium oxynitrides through the use of H2, CO, hydrocarbons or NH3.
The electroless deposition methods and pyrolytic methods used in conventional techniques such as described above produce large islands or dots of material deposited on the substrate material. Hence, a continuous coating is only obtained at the expense of depositing enough coating material to sufficiently coat the gaps between the islands or dots. This extensive deposition leads in turn to a relatively thick coating which, because of its thickness, does not generate the best chromatic colors.
Prior techniques for forming titanium-based coatings on a substrate have been limited to reducing an underlying titanium dioxide layer, resulting in a non-discrete layer interface. It is believed that the reduction of TiO2 layers results in high stress as the coating changes in chemical structure. As a result, voids may form in the coating if the new chemical structure requires less surface area and volume. Alternatively, bubbling may occur if the coating expands beyond its current surface area and volume because of a larger surface area being required to accommodate the change in chemical structure. The structural flaws degrade the optical qualities of the pigment.
Another difficulty with prior titanium coating techniques is that following the deposition of a titanium coating on a powdered substrate, the coated powder may auto-ignite by the spontaneous oxidization that can occur with a release of heat, as can happen during atmospheric venting of the vacuum chamber in a vacuum deposition process. Since the powder particles are poor conductors of heat, the heat is trapped and a runaway oxidation occurs which consumes the entire powder mass.
Accordingly, there is a need for alternative absorber materials and coating techniques which avoid the above drawbacks.